Various types of driver circuits are known having an output driven by a transistor controlled according to a signal on an input. In principle their output can drive a load coupled to a higher power supply (low side driver) or can drive a load coupled to a lower power supply (high side driver). They can have many different circuit implementations to suit a wide variety of applications.
An example of a classical low side driver has an output pin driven by an NMOS output transistor. Another NMOS transistor together forms a current mirror (with typically large mirroring ratio) which is driven by a transconductance stage Gm. Feedback is taken from the output pin through a resistive feedback divider. For high voltage applications the transistor can be an NDMOS type transistor. The output transistor can be cascoded by adding a further NMOS or NDMOS transistor in series with the output transistor.
The main disadvantage of this driver topology arises from the fact that such transistors have built in intrinsic drain bulk diodes. For normal operation these diodes are reverse biased so that current does not flow through them. But if the output voltage goes below ground, these diodes are activated allowing uncontrolled current flow through the output transistor drain bulk diode and the output. Due to this the circuit cannot provide protection against mis-wiring and missing ground faults.
Furthermore, during EMC disturbances the intrinsic diode of the output transistor can be activated. The charge moved out during the negative part of the disturbance period is much higher than the charge the driver is able to take during the positive part of the disturbance period (limited by the current capability (size) of the output transistor). This charge non-equivalency causes a positive DC level shift on the output during the EMC disturbance.
Drivers can be provided with a diode in series with output transistor to block the output current when the output pin is below ground. A diode in series with the output driver transistor is typical in vehicle networking drivers like CAN (Controller Area Network) or LIN (Local Interconnect Network). Although the EMC performance of this solution is much better (if the gain bandwidth (GBW) of the driver is much lower than the EMC disturbance frequency), the presence of the diode prevents the use of this solution for drivers with desired output voltages closer to the supply than one diode threshold voltage.
It is known from EP1280033 that traditional transconductance regulators are not EMC safe, thus electromagnetic interference will usually lead to instability of the output. Traditional solutions consist of adding filters in the input line for filtering out the EMC noise on this input signal. Such filters are very expensive and require external components. Hence this document proposes a voltage regulator circuit for providing a regulated output voltage at an output terminal, said regulator circuit comprising a current source (Icontrol) comprising a current source MOSFET, a current mirror circuit comprising a driver MOSFET, and a follower MOSFET interposed between said current source and said output terminal, operatively linked as to regulate an input voltage Vin to said regulated output voltage, and an EMC stabilizing MOSFET having its drain connected to its substrate and placed in series with any of said driver or follower MOSFETs. A PMOS with its bulk or substrate connected to the drain is used as EMC protection between the device to be protected and the node with the EMC disturbance. Any diode between the input supply and the regulated supply is thereby eliminated by means of an additional diode in an anti-series connection.
It is known from U.S. Pat. No. 7,119,999 to provide a reverse current blocking technique for a voltage regulator which does not employ PMOS, bipolar PNP, or external circuit current blocking devices to block the current flow when the supply connection is reversed. This document proposes a voltage regulator employing reverse current blocking via a lateral double-diffused MOS (LDMOS) device to block the current flow when the supply connection is reversed. By using an N-channel MOS transistor to block the current flow when the supply connection is reversed, less area is needed than if PMOS devices were used. When connected properly, the body diode conducts to provide a start-up function. A pre-regulator using the N-channel MOS transistor also employs a bias generator to enable the low drop-out voltage function, allowing the output voltage to be very close to the supply voltage.
It is known from EP0954079 that two NDMOS devices with anti-serially connected drain bulk diodes can be used for protection of output drivers which should achieve low output voltages. The protection transistor MP is operated as a switch, it is on during normal operation. When the output voltage goes below ground, a comparator detects this state and switches off the protection transistor MP. This document proposes circuits to enable faster switching off of the MP transistor in case the OUT voltage goes below ground. A disadvantage of this solution is a delay time needed for the circuit to switch off. Even if it is faster than the prior art, there is still remaining a delay of the comparator and the switch M. During the interval before the switch is switched off, the inverse current is on the order of several amperes. Another disadvantage of this circuit is a lower EMC performance due to the presence of this very high reverse current.